This invention relates generally to forming a joint between two mating components of an electrical circuit, and more particularly to using resistance welding between one electrical circuit component that includes axial leads to a shaped region of the other component as a way to reduce fracture-inducing stress risers.
Resistance welding (also referred to as electrical resistance welding) uses the electrical resistance of joined materials in conjunction with a high electrical current and high pressure force (both delivered through a pair of opposed electrodes) to maintain the materials together until a coalesced joint forms. Spot welding and seam welding are two well-known examples of resistance welding. Small-scale resistance welders (especially of the spot variety) are particularly well-suited for joining thin components, and as such are conventionally utilized for joining the wire-shaped axial leads of electrical components to flat substrates (examples of which include busbars, traces or terminals).
One common problem with resistance welding is that the high levels of force applied by resistance welder's electrodes to the axial leads tends to produce significant stress risers in the leads as they deform. In fact, such deformation of the lead may be significant, often reducing diameter of the wire by more than 50%. The reduced cross-section can leave a smaller amount of material in the interfacial region to carry the same load, making it susceptible to excessive loads and resultant fracturing during manufacturing processes as well as during the life of the system in which the assembly resides. The likelihood of such excessive stresses and attendant fracturing and premature failure is increased where the lead is exposed to vibratory forces and related fatigue-based dynamics, such as those encountered in automotive and related mobile environments.
The individual cells that make up an automotive battery pack deliver direct current (DC) electricity to various vehicle systems, including motors, electric traction systems (ETS) or the like. Many of these cells (which are incorporated into larger modules, packs or the like) may be electrically joined in series or parallel to achieve a desired level of respective voltage or current needed to power these automotive systems. Such joining takes place via highly conductive busbar or cabling assemblies, and may involve numerous interfacial regions formed between these and other system components.
Much of the joining takes place through resistance welding of small leads that may be exposed to the harsh mechanical and thermal loads associated with an automotive environment; the prolonged vibratory environment attendant to automotive travel is a particularly daunting challenge to the integrity of small leads, which are not robust enough to function as both a mechanical and electrical links between the electrically-joined components. This inability to survive over long-term usage (for example, years or even decades) means there is a significant probability of premature failure at the joined region.
Other joining approaches, such as the flex circuit-based approach and the use of rigid circuit boards, tend to use a large number of joints (in the case of the flex circuit) or bulky, rigid components (in the case of the circuit board); in either event, they involve complex manufacturing techniques, which make them prohibitively expensive, especially in high volume production oriented applications such as automotive battery packs.
As such, there is a need for a manufacturing process that provides greater robustness to joined electrical components, especially as it relates to their fatigue-resistance. There is also a need for joining such components over a wider range of welder-applied forces to allow for greater tolerances in the manufacturing process.